1. Field of the Invention
This invention relates to a device for taking a sample of molten metal for making a determination of the amount of hydrogen therein and an apparatus for the determination of the amount of hydrogen in molten metal.
2. Description of the Prior Art
In order to produce steel or aluminum having good mechanical properties, it is essential to reduce the quantity of hydrogen in the molten metal as far as possible. The determination of the amount of hydrogen in molten metal has conventionally been carried out by taking a sample in a quartz tube directly from the molten metal or with the aid of a spoon, quenching it with water, cutting the solidified sample to an appropriate size immediately or after storing the sample in dry ice or liquid nitrogen, polishing the surface, and analyzing the sample by a method which includes melting in an inert gas, gas chromatographic separation and thermal conductivity determination. A method including vacuum heating and constant volume pressure measurement is also employed for hydrogen determination.
It has, however, been difficult for a number of reasons to conduct an accurate determination of the amount of hydrogen in molten metal. In the first place, the solubility of hydrogen is greatly lowered with the solidification of the molten metal, as shown in FIG. 1 showing by way of example solubility in iron. A large quantity of hydrogen is lost into the ambient air during the solidification of the molten metal, and makes impossible the proper determination of the amount of hydrogen in the molten metal. In the second place, the loss of hydrogen also takes place during the cutting or polishing of the solidified sample, since it is supersaturated with hydrogen at an ambient temperature.
A number of proposals have been made to solve those problems. The first proposal has been to use a vacuum quartz sampling tube containing a stainless steel cylinder having a small wall thickness such that the hydrogen which is released from the molten metal during its solidification is absorbed by the stainless steel in which hydrogen is highly soluble, and then the solidified metal and the stainless steel cylinder are cut together to form a sample (Narita et al., IRON AND STEEL, 65, 1979, 1620). This method has, however, a serious disadvantage. A very small clearance is very likely to be formed between the solidified metal and the stainless steel cylinder, and traps water when the sample is cooled with water. This water is decomposed when the sample is melted for analysis, and the resulting hydrogen causes an error in the determination of the amount of hydrogen in the molten metal. This error is aggravated unless the stainless steel cylinder is completely dehydrogenated previously.
The second proposal relates to a sampler which comprises a hollow body formed by a thin metal wall 1 and caps 3 and a sampling mold 3 contained therein, as shown in Japanese Patent Publication No. 45157/1978. The hollow body is hermetically closed and defines a vacuum chamber 2-1 therein. The cap 2-1 is immersed in molten metal and a sample of the molten metal is drawn into the mold. The sampler is cooled so that hydrogen is released from the molten metal and collected in the vacuum chamber. The greater part of the sampler remote from the cap is placed in a vessel connected to a gas analyzer, and the inlet of the vessel is closed. The wall is pierced to release hydrogen from the vacuum chamber to the analyzer. If the entire sampler is placed in the closed vessel and its wall is pierced with a hole, a part of the hydrogen released from the metal on the suction end of the sampler will also enter the analyzer and give rise to an error in the determination of the amount of hydrogen. This method has the advantage of avoiding the loss of hydrogen which would otherwise occur during the solidification of the sample, or during its cutting or polishing which is no longer necessary. The method has, however, a number of disadvantages, too, as will hereinafter be pointed out:
(1) The sampler is covered by a heat-insulating material which prevents the melting of the thin metal wall 1 when it is immersed in molten metal. The heat-insulating material contains an organic binder. When the sampler is immersed in molten metal, the binder burns and causes the molten metal to boil and draw air thereinto. A part of the heat-insulating material is melted to form slag which increases the quantities of C, H, O, Si and inclusions in the molten metal. These problems make it impossible to use the sampler in a tundish or a mold immediately prior to continuous casting, and make it possible to use the sampler only at an earlier stage.
(2) If the heat-insulating material is removed, it is impossible to avoid the adhesion of molten metal to the outer periphery of the wall and the cap 2-1, even if the sampler is immersed in molten metal for only a short period of time which does not cause any melting of the wall. The metal adhering to the sampler makes it very difficult to achieve a satisfactory seal between the sampler and the vessel and makes any accurate hydrogen determination practically impossible.
(3) A rubber O-ring is usually used to provide a seal between the vessel and the sampler. After a predetermined amount of molten metal has been drawn into the sampler, it is cooled with water so that the O-ring will not be burned. In the case of hydrogen analysis in steel, about 5 to 10% of hydrogen remains in the steel in the mold without being released into the vacuum chamber. Therefore, it is necessary to cut the mold and determine the remaining hydrogen by a hot extraction hydrogen analyzer after determining the amount of hydrogen in the vacuum chamber, and make a total of the two values obtained. This is a serious obstacle to quick analysis.